The embodiments herein relate to methods for producing fluid migration resistant cement slurries.
Subterranean formation operations (e.g., stimulation operations, sand control operations, completion operations, etc.) often involve placing a cement sheath around a casing (or liner string) in a wellbore. The cement sheath is formed by pumping a cement slurry through the bottom of the casing and out through an annulus between the outer casing wall and the formation face of the wellbore. The cement slurry then cures in the annular space, thereby forming a sheath of hardened cement that, inter alia, supports and positions the casing in the wellbore and bonds the exterior surface of the casing to the subterranean formation. This process is referred to as “primary cementing.” Among other things, the cement sheath may keep fresh water zones from becoming contaminated with produced fluids from within the wellbore. As used herein, the term “fluid” refers to liquid phase fluids and gas phase fluids. The cement sheath may also prevent unstable formations from caving in, thereby reducing the chance of a casing collapse and/or stuck drill pipe. Finally, the cement sheath forms a solid barrier to prevent fluid loss or contamination of production zones. The degree of success of a subterranean formation operation involving placement of a cement sheath therefore depends, at least in part, upon the successful cementing of the wellbore casing.
Cement slurries are typically designed to have a hydrostatic pressure between the formation pore pressure and the fracture gradient of the formation to prevent fluid migration within the cement slurry and prevent fracturing of the subterranean formation. As used herein, the term “formation pore pressure” refers to the pressure of the subsurface formation fluids within the subterranean formation itself. As used herein, the term “fracture gradient” refers to the pressure required to induce or enhance fractures in a subterranean formation at a given depth. During cement hydration, the hydrostatic pressure of the cement slurry decreases and it may drop below the formation pore pressure, allowing fluid invasion and migration within the cement slurry, a common obstacle of primary cementing. As used herein, the term “hydrating cement slurry” refers to a cement slurry that has not fully hydrated and become a solid, hardened mass.
Fluid migration can present significant economic and environmental challenges. For example, fluid may migrate through channels within the hydrating cement slurry to a lower pressure portion of the slurry or to the surface of the subterranean formation. As used herein, the term “channel” refers to a defect in the quality of cement, where the cement does not fully occupy the annulus between the casing and the formation face. The migration may result in substandard performance of the cured cement sheath resulting in failure of zonal isolation or wellbore structure failure. Failure of zonal isolation could result in environmental contamination, which may cause harm to both flora and fauna, including humans. The pressure created by the fluid migration may also lead to a well blowout. Because of the potentially costly effects of fluid migration on a cement sheath, both in economic and environmental terms, a number of methods have been established to evaluate the potential of fluid migration within a hydrating cement slurry. These methods focus solely on the hydration kinetics profile of the cement slurry itself. As used herein, the term “hydration kinetics profile” refers to the time required to fully hydrate the cement slurry into a hardened sheath. Specifically, the hydration kinetics profile depends upon any property of the cement slurry that contributes to the curing, strength, and hydrostatic pressure of the cement slurry within a subterranean formation (e.g., compressibility, shrinkage, shear rate, and rheological properties of the cement).
However, potential of fluid migration into a hydrating cement slurry is also dependent upon the fluid migration threshold of the cement slurry. As used herein, the term “fluid migration threshold” or “fluid migration threshold pressure” refers to the critical pressure required to cause a break (e.g., cause crack propagation of a fluid or a fluid bubble) in a hydrating cement slurry. Typically, crack propagation occurs when an outside pressure exceeds the sum of the horizontal and tensile stress pressures exerted by the hydrating cement slurry at a particular time. As used herein, the term “crack propagation” refers to a fluid migration profile in which the migrating fluid creates a more or less longitudinal or lengthwise artery, conduit, or channel. Crack propagation of a fluid bubble within a hydrating cement slurry may be compared to a worm burrowing through a cohesive marine sediment or a gelatin substance. The burrowing worm creates a stress field dorsal and anterior to itself and a more or less longitudinal or lengthwise artery or conduit extending anteriorly to its body. The crack propagation of a fluid bubble may also be compared to methane bubble growth in cohesive marine sediment.
A comprehensive method of predicting fluid migration potential after primary cementing in order to produce a fluid migration resistant cement slurry would be beneficial to one of ordinary skill in the art.